8" is about the max you can go without changing any cables. I have 10" Z bars and only had to replace the front brake line. I cut a couple cable ties and moved the other cables above the front tank bolt mounts to gain a little more length.
I did 10" without issue (8" bars with 2" risers). As was mentioned above, the brake line was at it's max (had to be unbent). I ran like that all last summer and just swapped out the brake cable this winter. No issues.
Everything these guys have already said, plus, no matter what bars you go with that solid part of the front brake line will need to be unbent in some way.
You can just straightend it by hand. We did that on my brothers bike cause he was to cheap to by any new parts. Any tube and hose place can make you a brake line for cheap